I had a '78 Camaro with the 5" rear spoiler do 25 MPG at 65 MPH with the centrifugal advance not working, and a mystery Q-Jet. That was a 350, I swapped a junkyard 700R-4, it had a 3.08:1 axle, and I still have every detail to duplicate it, but the point is, hoping for 15 is a joke. It should already do that just as you got it.